Coal processing, particularly the treatment of metallurgical coal, usually incorporates washing steps as part of the beneficiation scheme. Coal washing is primarily to remove undesirable gangue minerals from the coal, usually by froth flotation. The water used in the coal cleaning process must be removed for a number of reasons. Firstly, there are usually moisture-limit specifications on coal sold to industry by the coal producers. Moisture in coal results in increased freight costs and reduced thermal-energy value. Secondly, in many parts of the world, shipment of wet coal in winter results in problems in freezing of the coal in rail cars. Thirdly, refuse coal must be dewatered prior to disposal.
The coal-cleaning process produces coal with a broad range of particle sizes. Coarse coal (i.e., &gt;600 micron) and fine coal are treated differently to dewater effectively. Water is usually removed from coarse coal by screen drainage or centrifugation and from fine coal (i.e., flotation concentrates or refuse) by centrifugation or vacuum filtration. In some cases, thermal drying of fine coal is employed as a necessary step to achieve target moistures. Depending upon the mining operation, the type of coal treated and normal swings in process operations in the plant, the moisture levels in the cleaned coal can vary quite widely. Typical moisture levels in coarse coal range from 2-12%, while fine coal moistures can range from 15-30%.
Coal processing plants often have considerable trouble meeting target moisture levels in the product coal they sell. As a result of new mining methods employed in long-wall operations, the proportion of fine coal reporting to the prep plants has increased significantly. Fine coal is much more difficult and costly to dewater than coarse coal. One method often employed by coal prep plants to achieve moisture specifications is to blend high-moisture fine coal with low-moisture coarse coal in proportions necessary to just meet target moisture. While, in many cases, it would be much more cost effective for a coal prep plant to simply discard the fines and mine more coarse coal, for reasons of resource management/utilisation/conservation it is more prudent to treat the fines.
Vacuum filtration is the most commonly used means of mechanical treatment to dewater fine coal. Fine coal, in slurry form, reports to the filtration operation where the water is removed. Vacuum disc filters are the principal type used by the coal industry to filter fine coal, although vacuum belt filters are being utilised on an increasing basis. To assist in the efficient operation of vacuum filters, reagents are often added to the feed slurry.
The coal industry has been using anionic flocculants and cationic coagulants in the vacuum filtration of fine coal for a long time. These reagents are necessary to `thicken` the slurry as it is fed to the vacuum filter to ensure the proper formation of a filter cake. It is thought that the reagents function by binding the very fine coal particles to larger coal particles in a typical flocculation/coagulation process, thus producing a more uniform particle size distribution in the filter cake. This results in better permeability of the filter cake and less `blinding` of the filter cloth by the very fine coal particles.
Both anionic flocculants (usually high molecular weight acrylamide/acrylate co-polymers) and cationic coagulants (usually low molecular weight polyamines) are used, individually or in combination, to control filter cake formation. These reagents are always added to the slurry feeding the vacuum filters (i.e., slurry pre-treatment) in a manner typical of flocculant/coagulant addition in the minerals industry.
There has also been considerable interest by the coal industry in the use of surfactants, in combination with flocculants/coagulants, to enhance the dewatering of fine coal. Although many studies have been conducted and reported, there are widely varying results/conclusions. Some studies have shown that surfactants do not significantly affect residual cake moisture, while others show they do. Other studies conclude that surfactants can be effective for certain types of coal but not for others.
One of the most significant reasons for the variation in the performance of dewatering aids for fine coal is the variable chemistry of coal itself. Unlike all other minerals, coal is an organic material. Coals vary widely in bulk/surface composition, depending upon a multiplicity of factors such as location of the deposit, rank and mineral (inorganic) matter, degree of weathering, internal structure/porosity, etc. Therefore, depending upon the specific coal treated, dewatering aid performance can be expected to vary widely since both the adsorption (a surface chemical property) and absorption (a bulk chemical property) characteristics of coals with respect to drainage aid interaction will vary widely.
Regardless of the conclusions in the various studies related to the performance of dewatering aids, virtually every published fine-coal filtration study has concluded that surfactant utilisation to improve fine-coal dewatering is uneconomical. Surprisingly, most of the studies reported have limited the surfactants examined as dewatering aids for fine coal to detergents such as ethoxylated alcohols, alkyl sulfosuccinates, and alkyl sulfates. These are the most commonly used surfactants as drainage aids by the minerals industry but they are expensive and adsorb strongly onto coal surfaces. Surfactant adsorption increases reagent demand and is usually the most significant cause of cost inefficiency of dewatering aids for mineral filtration. United States patent specification U.S. Pat. No. 4,447,344 (Roe-assigned to Nalco Chemical Company) discloses that a blend of a nonionic with a hydrotrope (i.e., sodium xylene sulfonate) eliminates the problem of surfactant loss to the coal surfaces, in practise the technique does not achieve the objects of the present invention and excessively high reagent dosages are required.
United States patent specification U.S. Pat. No. 4,231,868 (Wang et al assigned to American Cyanamid Company) discloses surfactant and/or surfactant compositions specifically "designed" for improved coal dewatering.
However, despite the claims made as to the cost efectiveness of the reagents disclosed, one of these patents (U.S. Pat. No. 4,231,868) specifically recognises the need for improved (e.g., less costly) reagents.
It has been described in European Patent Application EP-A-0,417,360 that the dewatering of alumina trihydrate obtained by the Bayer process is improved by treating the alumina trihydrate with the product of mixing an alkaline liquor and a C.sub.8 -C.sub.20 fatty acid or fatty acid precursor. As an example at least 5 grams/tonne of alumina trihydrate filtered of oleic acid is used. In this reference there is no indication or suggestion to use this system for coal dewatering and also the physico-chemical properties of alumina trihydrate particles are different from those of coal particles.
In American Patent U.S. Pat. No. 4,410,431 (Nalco Chemical Comp.) it has been proposed to alter the water function characteristics of coal particles by the application of a mixture of a surfactant material having an HLB number of 6.0-12.0 and a surfactant adsorption inhibitory amount (10-65 wt % of the mixture) of a fatty acid, like tall oil fatty acid. There is no indication or suggestion that in the absence of a surfactant excellent results may be obtained.
In East German Patent DD-A-0,120,801 (Koch et. al.) it has been proposed to use saturated or unsaturated fatty acids to improve the filtration of suspensions. The active agent can be added to the suspension and/or the washing water. As an example the filtration of sodium bicarbonate has been mentioned, but nowhere the use in coal filtration has been mentioned or suggested.
Finally, it has been proposed in Japanese Patent Application JP-A-56-072,083 (Nippon Oils & Fats KK) to separate water from a coal-water slurry in a two-stage process. In the first stage 10-3000 ppm of an anionic surface active agent, e.g. fatty acid soap, is added to a coal-water slurry with at most 80% by weight of coal particles having a particle size of at most 5 mm, after which the mixture is filtered or centrifuged. In order to remove sufficient water, however, a second treatment of the coal with a hydrocarbon, like kerosine, is required.
It is the object of this invention to provide a method and a dewatering aid to provide effective and economical coal dewatering treatment using inexpensive fatty acids or their derivatives without the need for expensive surfactants and/or emulsifiers. Fatty acids themselves are not considered to be surfactants because of their extremely low aqueous solubility.